Internal combustion engines generally produce engine output torque by performing combustion in the engine cylinders. Specifically, each cylinder of the engine inducts air and fuel and combusts the air-fuel mixture, thereby increasing pressure in the cylinder to generate torque to rotate the engine crankshaft via the pistons. One method to improve engine fuel economy during deceleration is to deactivate fuel injection to all or a selected group of cylinders to thereby reduce combustion torque and increase engine braking.
The above approach can provide engine braking from engine friction and pumping work (due to manifold vacuum). The compression and expansion of air in the cylinders during the compression and expansion stroke results in energy storage and recovery, and thus may not contribute to engine braking. As such, one approach to increase engine braking is referred to as a “Jake Brake”. A Jake Brake opens the exhaust valve at top dead center of compression, thereby reducing or eliminating the energy recovery of the expansion stroke. This, in turn, can increase engine braking significantly since the unrestrained expansion is dissipating energy stored during the compression stroke.
One approach to incorporate Jake Brake type engine braking is described in U.S. Pat. No. 6,192,857, in which exhaust valve timing is adjusted to control a level of engine braking provided. See also FIG. 20 herein. However, in the approach of '857, the early opening of the exhaust valve may produce flow through the exhaust system of oxygen rich gas during fuel-cut operation. This can degrade catalyst performance due to cooling and saturation, while also increasing emitted noise due to unrestrained expansion of gasses into the exhaust system.
An approach to reduce airflow during fuel-cut operation is described in U.S. Pat. No. 6,526,745, in which at least one (or both) of the intake or exhaust valve is placed in a closed state to block any flow through the engine. However, while this may reduce airflow through the exhaust, engine braking effects may be lost (or significantly reduced). In other words, if there is no air flowing through the engine, engine braking due to pumping work is reduced or lost. Further, since there is no indication of any expansion or compression work being performed, engine braking may be significantly reduced.
The inventors herein have recognized the above issues. And, faced with the paradoxical approach of the prior art (where either engine braking may be obtained at the expense of catalyst performance, or catalyst performance may be maintained at the expense of engine braking), the inventors herein have developed various systems and approaches that attempt to reduce at least some of the above tradeoffs.
In one example, a method for operating a cylinder with at least an intake and exhaust valve, the engine in a vehicle, may be used. The method comprises maintaining at least the exhaust valve in a closed position during a period of net engine torque less than zero, and during said period of net engine torque less than zero, operating with at least the intake valve open, then closing the intake valve, and then opening the intake valve.
In this way, it may be possible to provide engine braking while reducing net flow through the engine. In other words, since the exhaust valve is maintained closed, flow is impeded from the intake to the exhaust, or vice versa. And, the intake valve may be operated to provide expansion or compression braking in the cylinder, for example, in which flow may enter and exit the cylinder through the intake valve. In this way, desired engine braking can be obtained even when there is reduced braking from engine reduced engine pumping work.
In one specific example where a throttle is used, by adjusting intake valve timing to generate braking (expansion or compression type), reduced noise may be achieved due to closing (fully or partially) the throttle plate. Also, by adjusting intake valve timing to generate braking, it may be possible to enable use of a mechanical exhaust valve system, if desired.
Note that the opening of the intake valve can be either full or partial opening. Also note that the period can be an expressly defined period, or a variable period, for example. Further, conditions of net engine torque less than zero may be conditions where torque of the engine is actively controlled to be negative, or conditions that result in such a situation, among other conditions, for example.